Fullerenes are a recently discovered class of multi-atomic closed cage carbon clusters with the truncated icosahedron C60, buckminsterfullerene, being the most abundant. Several methods of synthesizing fullerenes are known. The electric-arc method is a common method of making fullerenes. However, current electric-arc methods are not typically suitable for commercial-scale production of fullerenes due to high cost and low production rates. A more commercially promising method of making fullerenes is the combustion method.
The combustion method for fullerene production is a continuous, scalable process that employs inexpensive hydrocarbon fuels, including among others aromatic and/or aliphatic hydrocarbons. As such, it is the most economical known route for large-scale commercial production of fullerenes. Fullerenes and fullerenic soot have potential applications as additives to electron and photo-resists for semiconductor processing; for use in proton-conducting membranes for fuel cells, optical limiting materials and devices, and lithium battery anodes; as active elements in organic transistors; as pigments in cosmetics; as antioxidants; and as therapeutics, e.g., as anti-viral agents. Many of these applications are sensitive to impurities such as PAHs. In some applications, particularly in pharmaceuticals and cosmetics, PAHs must be removed because of their known carcinogenicity. In other applications, small amounts may be tolerable without affecting the application, but would not be acceptable for marketing and/or liability concerns. In either case, the PAHs and other hydrocarbon contaminants are a hindrance to acceptance of the combustion process for fullerene synthesis and could potentially offset its favorable cost advantage.
Carbon nanomaterials, a broader class of potentially useful materials which includes soluble and insoluble fullerenes, as well as single-walled carbon nanotubes (SWNTs), multiple-walled carbon nanotubes (MWNTs), nanotubules, and nested carbon structures with dimensions on the order of nanometers, can also be produced in soot from combustion methods and can be contaminated with undesirable impurities including various PAHs. Combustion soot containing carbon nanomaterials, fullerenes or both is useful in various applications.
The types and amounts of PAHs produced during carbon nanomaterial synthesis can depend upon the fuel used and the combustion conditions. The electric-arc process typically generates fewer PAH and hydrocarbon impurities than combustion processes. Thus, the ability to efficiently remove PAHs and other hydrocarbon contaminants from carbon nanomaterial containing combustion products is important for keeping the combustion production process competitive with the arc process.
Solvent washing has been used for cleaning fullerenes produced in the electric-arc process. The first mention of this method is in the original report of bulk C60 production by Kratschmer et al. (1990), “Solid C60: a New Form of Carbon,” Nature 347:354. Ether was used to remove “ubiquitous hydrocarbons” from the soot prior to either dispersing the soot in benzene to form a solution containing C60 or heating the soot to sublime the C60. The “ubiquitous hydrocarbons” were, however, not specifically identified. Since the electric-arc process produces very little, if any, PAH impurities, it is unlikely that these “ubiquitous hydrocarbons” contained substantial quantities of PAHs.
The ability to wash various non-aromatic impurities from combustion synthesized fullerenes using diethyl ether was noted by McKinnon (McKinnon et al. 1992, “Combustion Synthesis of Fullerenes,” Combustion and Flame 88:102). McKinnon et al. treated their combustion soot with toluene, then treated the soot extract with diethyl ether. McKinnon et al. expected to find oxygenated PAH impurities in the soot toluene extract, but no aromatic impurities were identified. Infrared (IR) analysis of the toluene raw extract indicated the presence of a “carbonyl material” impurity which was not aromatic. The “carbonyl material” appeared by IR to be largely removed from the fullerene material by a diethyl ether wash. The carbonyl material was not further identified.
A paper by Zakharov et al. (2000) “Electronic Absorption Spectra Determination of C60 Content in Soot Produced in Hydrocarbon Flames,” J. Appl. Spectrosc. 67:349, reports experiments in using electronic absorption spectroscopy to measure the content of fullerenes. The authors report methods for lowering the content of unidentified hydrocarbon compounds in combustion soot which may interfere with and decrease the accuracy of the determination of fullerene content by electronic absorption spectroscopy. The authors generated soot employing a method that was expected to generate fullerene-containing soot. However, the soot generated was shown by mass spectral analysis not to contain fullerenes (see page 351, first full paragraph, lines 6-7). Thermal treatment or ether treatment of this soot was reported to remove hydrocarbon impurities and reduce the electronic absorptions that would be expected to interfere with measurement of electronic absorptions characteristic of fullerenes. Thermal treatment involving heating the soot sample at temperatures of 374 K, 498 K, or 593 K was reported to be more effective then ether treatment for decreasing the potentially interfering absorptions. Ether treatment involved pouring a 20 mg soot sample into 20 ml of ether, leaving the soot sample in contact with the ether for 30 to 120 minutes, then decanting the ether. In some cases the ether treatment procedure was repeated. The methods described, however, were not demonstrated to selectively remove undesired impurities from fullerene-containing soot, because the soot employed in the experiments reported did not contain fullerenes.
The Inventors of the instant-invention have discovered an extraction process specifically suitable for cleaning PAHs and other hydrocarbon impurities from combustion-produced carbon nanomaterials, including fullerenes and fullerenic soot. With this discovery, a new process for purifying carbon nanomaterials from combustion soot can be economically run on kilogram quantities of combustion soot.